Aqueous paints are excellent in terms of environmental protection and worker safety and so they are taking over from the solvent type paints. For example, a change from the solvent type metallic paints in which large amounts of organic solvent are used to aqueous metallic paints in which water is used for the solvent is in progress in the field of the metallic paints which are used to paint the outer panels of automobiles.
Random polymers which have carboxyl groups in the amounts required to make them compatible with water exist as aqueous resins which can be used generally in aqueous paints or aqueous paint compositions, and aqueous dispersions in which these polymers are dispersed in an aqueous medium and neutralized with a basic substance are being used as aqueous paints.
However, although the aqueous paints which consist of such an aqueous dispersion, for example the aqueous paints comprising an aqueous dispersion of acrylic based random polymer, have good pigment dispersion properties and paint fluidity and form paint films which have an excellent gloss and smoothness and they are suitable for providing a beautiful finish, the acrylic based polymers have a high acid value and so the water resistance of the paint film is low. Furthermore, the viscosity of an aqueous dispersion is high and so, when the aqueous paints obtained from them are adjusted to a viscosity which is suitable for painting, the solid fraction concentration becomes very low, and defects such as runs, holes and unevenness, for example, are liable to arise in the paint film during painting and during drying.
Aqueous paint compositions which contain acrylic based graft copolymers comprising hydrophilic components and hydrophobic components have been proposed for providing some improvement in these respects. For example, the manufacture of an acrylic based graft copolymer of acid value 15-40 and glass transition temperature -10.degree. C.-70.degree. C. by polymerizing an unsaturated group-containing acrylic based polymer of acid value 40-200, obtained by reacting a glycidyl group-containing unsaturated monomer with a carboxy functional acrylic based polymer, and a monomer composition in which .alpha.,.beta.-ethylenic unsaturated carboxylic acid and other copolymerizable unsaturated monomers have been compounded in such a way that the acid value is not more than 30, the difference in the acid values of the two reaction components being 25-200, and aqueous paint compositions which contain aqueous dispersions of the acrylic based graft copolymers obtained as resin components, have been disclosed in Japanese Patent Kokai 56-49760.
Glycidyl group-containing unsaturated monomers are first reacted with a carboxy functional acrylic based copolymer in the method for the manufacture of the abovementioned acrylic based graft copolymers and the glycidyl groups undergo ring-opening addition with the carboxyl groups, and then the monomer composition is graft polymerized. However, in this method, the glycidyl groups only react specifically with carboxyl groups and so graft copolymers cannot be obtained from acrylic based copolymers which do not contain carboxyl groups, and so aqueous paints cannot be obtained. Furthermore, improvement in the painting operability is desirable with the aqueous paints which contain the abovementioned acrylic based graft copolymers.